Aliasing effects occur due to bad sampling of scan lines over the object. This occurs due to discontinuity of objects and scanning is restricted at integer values.
During the scan conversion, same pixel may be unneccesarily plotted multiple times which causes overstriking.
Unequal intensity occurs due to uneven brightness of the object during scan conversion.
Scan converting object normally into the image space will result in uneven distances between pickets since the endpoints will have to be snapped to pixel coordinates. This is sometimes called global aliasing, as the overall length of the picket fence is approximately correct. On the other hand an attempt to maintain equal spacing will greatly distort the overall length of the fence. This is sometimes called local aliasing, as the distances between pickets are kept close to their true distances.
Hence option (4).